Problems with fogging have plagued the photographic industry from its inception. Fog is a deposit of silver or dye that is not directly related to the image-forming exposure, i.e., when a developer acts upon an emulsion layer, some reduced silver is formed in areas that have not been exposed to light. Fog can be defined as a developed density that is not associated with the action of the image-forming exposure, and is usually expressed as "D-min", the density obtained in the unexposed portions of the emulsion. Density, as normally measured, includes both that produced by fog and that produced as a function of exposure to light. It is known in the art that the appearance of photographic fog related to intentional or unintentional reduction of silver ion (reduction sensitization) can occur during many stages of preparation of the photographic element including silver halide emulsion preparation, spectral/chemical sensitization of the silver halide emulsion, melting and holding of the liquid silver halide emulsion melts, subsequent coating of silver halide emulsions, and prolonged natural and artificial aging of coated silver halide emulsions.
The control of fog, whether occurring during the formation of the light-sensitive silver halide emulsion, during the spectral/chemical sensitization of those emulsions, during the preparation of silver halide compositions prior to coating on an appropriate support, or during the aging of such coated silver halide compositions, has been attempted by a variety of means. Mercury-containing compounds, such as those described in U.S. Pat. Nos. 2,728,663; 2,728,664; and 2,728,665, have been used as additives to control fog. Thiosulfonates and thiosulfonate esters, such as those described in U.S. Pat. Nos. 2,440,206; 2,934,198; 3,047,393; and 4,960,689, have also been employed. Organic dichalcogenides, for examples the disulfide compounds described in U.S. Pat. Nos. 1,962,133; 2,465,149; 2,756,145; 2,935,404; 3,184,313; 3,318,701; 3,409,437; 3,447,925; 4,243,748; 4,463,082; and 4,788,132 have been used not only to prevent formation of fog but also as desensitizers and as agents in processing baths and as additives in diffusion transfer systems.
Unfortunately, such fog reducing compounds are not without drawbacks. Mercury-containing compounds at relatively low concentrations can diminish the sensitivity of silver halide emulsions, can cause a deterioration in the stability of the latent image, and are environmentally harmful. The elimination of mercury-containing compounds from photographic compositions is highly desirable. Thiosulfonate salts can cause large sensitivity losses if not used with an excess of sulfinate salt. Many of the very effective organic disulfide compounds need to be added to silver halide compositions from typical organic solvents because of their high water insolubility. This has several drawbacks, for example, coagulation of hydrophilic binders by the solvents, rapid flocculation and crystallization of the disulfide compounds when added to aqueous melts, solvent explosion hazards, and the potential for environmentally harmful solvent emissions.
One method used to overcome some of the problems associated with organic disulfide compounds is to prepare a solid particle dispersion, such as described in U.S. patent application Ser. No. 07/869,678, Boettcher et al., filed Apr. 16, 1992. However, this method, while effective, requires the additional steps and costs of dispersion preparation, certification, and handling. Further, the use of gelatin-containing dispersions has limitations when the presence of gelatin is undesired, such as during the precipitation of silver halide emulsions in synthetic protective colloids.
Some water-soluble disulfide compounds have been described for use with photographic elements. U.S. Pat. Nos. 3,057,725 and 3,062,654 describe substituted alkylene disulfides as antifoggants in silver halide emulsions. U.S. Pat. No. 3,226,232 describes the free acids of carboxyphenyl disulfides as antifoggants and stabilizers in silver halide emulsions. U.S. Pat. No. 3,597,207 discusses bis(loweralkylcarbonamidophenyl) disulfides for use in pretreatment solutions for processing color reversal compositions. U.S. Pat. No. 3,761,277 discusses sulfophenyl-substituted disulfides as fog stabilizers. U.S. Pat. No. 3,779,757 describes aromatic acid-substituted disulfides for diffusion transfer as image toners. U.S. Pat. No. 3,926,632 broadly describes a class of disulfides which are utilized solely in fine-grained lith materials. U.S. Pat. No. 4,521,508 describes aromatic or heterocyclic disulfides as additives in internally fogged silver halide emulsions. U.S. Pat. No. 4,699,873 describes the disulfides of U.S. Pat. No. 4,521,508 in combination with hydrazines in high-chloride-content graphic arts material.
None of the disulfide compounds known in the art, however, have the combined attributes of high water solubility and strong antifogging activity with de minimus impact on sensitivity. A photographic additive with these qualities is highly desired in the photographic industry.